The invention relates to bonding a metal brush structure to a planar surface of a metal substrate.
One of the difficulties in the fabrication and use of bonded joints of metal materials is accommodating any differences in the respective coefficients of thermal expansion (CTE). CTE is also often referred to as the coefficient of linear expansion, and these terms have interchangeable meaning. The difference in CTE's of bonded materials can result in stresses being exerted at the bondline during thermal cycling and after joinder at elevated temperatures. These stresses can reduce the service life, and even cause failure of the joints. The joint failures are especially prone to occur during cool-down from the initial joining temperature or in the course of heat cycles during service.
A method of reducing these stresses has involved the use of what is known as a "brush" structure in which one of the two materials is configured as small metallic filaments, which effectively reduces the strain, and thus, the stresses at the edge of the joint.
An inter-metal bonding system of interest in nuclear fusion research in particular is that of copper to tungsten. As illustrated in FIG. 1, in the nuclear fusion research milieu, the application of copper-tungsten bonds involves joining an exterior surface of a water-cooled copper alloy heat sink 10 to a tungsten tile 12, referred to occasionally herein as "armor," to protect the copper alloy material constituting the heat sink 10 from the sputtering erosion and occasionally extreme temperatures that can be generated by the reactor. The copper heat sink 10 has one or more interior water channels 11 that permits water to flow through the heat sink 10 for heat exchange and cooling purposes. In this context, the "brush" structure can be incorporated into tungsten tile 12. However, prior attempts made at fabricating such a structure using thin wires in the construction of tungsten brush tile 12 have not been entirely successful, and the cost have been prohibitive from a production standpoint, due in part to the handling problems associated with the use of fine wires. Also, the presence of copper throughout the brush structure has proven onerous because the copper must be etched or machined out prior to service.